Water heaters for supplying hot water to a bathroom, kitchen, or another place can generally be categorized into types such as electric water heaters, gas water heaters, and oil-fired water heaters, all of which include a part called a heat exchanger to transfer heat to water. Among these water heaters, electric water heaters of the heat pump heat exchanger type (heat pump water heater) in particular have recently been of interest from the standpoint of carbon dioxide reduction as measures for energy conservation and against global warming.
The principle of a heat pump water heater is that atmospheric heat is transferred to a heat medium and water is boiled with the heat. Specifically, the principle is based on the repetition (cooling energy cycle) in which high temperature heat generated by compressing a gas is transferred to water via a heat exchanger and, using the chill generated by expanding the gas, the temperature of the heat medium is returned to atmospheric temperature. Theoretically, heat energy greater than the input energy cannot be extracted, but with a heat pump water heater, which is configured to utilize atmospheric heat, it is possible to use heat energy greater than the energy required for operation.
A very important issue for the heat exchanger in a heat pump water heater is to keep the heat transfer surface clean consistently to transfer heat to water. Contamination on the wall surface of the heat transfer surface reduces the effective heat transfer surface area, resulting in decreased heat transfer performance. Further, accumulation of contaminants increases pressure drop that occurs with the water flow into the heat exchanger, and in the worst case, can cause flow path blockage. One problem encountered particularly in districts where a concentration of hardness components in water (such as calcium ions, magnesium ions, ionic silica, and molecular silica) is high is that a less soluble inorganic compound salt called scales (scale particles) precipitates as a result of heating and deposits in the heat exchanger.
One mechanism by which hardness components in water deposit as scales on the inside of a heat exchanger is that nuclei of calcium carbonates are formed on the high temperature surface such as the heat transfer surface of the heat exchanger and crystal growth of scales progresses. Also, in the case where the water in the tank of a water heater is heated while the water is circulated through the heat exchanger by a pump, the nuclei or particles of calcium carbonates formed in the hot water heated by the heat exchanger may be reintroduced to the heat exchanger and deposit on the heat transfer surface.
Patent Literature 1 discloses a method for separating scale particles as described above from water. This method for preventing scale deposition onto a heat exchanger is that heated water heated by a heat exchanger device is caused to pass through a scale trapping unit that incorporates a scale adsorbing element having a function of adsorbing scales. Patent Literature 1 also discloses a technique of renewing the adsorbing function of the adsorbing element by introducing tap water into the scale trapping unit as rinse water during a time period in which heating is not performed to remove scales from the scale adsorbing element and discharge the scales to the outside of the water circuit.
Patent Literature 2 discloses a method for preventing scale deposition onto a heat exchanger. The method is that an electrode-type scale component precipitation device having a potential difference of at least 2 V is connected between pipes at the water inlet side, and a scale component trapping unit is provided downstream of the scale component precipitation device, the scale component trapping unit designed to trap scales precipitated in the electrode-type scale component precipitation device.